Читать книгу A Text-book of Entomology - A. S. Packard - Страница 48

Farther observations are needed to connect the mode of formation of the wings in the holometabolous insects with the more primitive mode of origin seen in the hemimetabolous orders, but the former mode is evidently inherited from the latter. Pancritius remarks that the development of the rudiments of the wing in a hypodermal cavity is in the holometabolic insects to be regarded as a later inherited character, the external conditions causing it being unknown.

Fritz Müller was the first to investigate the mode of development of the wings of the hemimetabolic insects, examining the young nymphs of Termites. He regards the wings as evaginations of the hypodermis, which externally appear as thoracic scale-like projections, into which enter rather late in nymphal life tracheæ which correspond to the veins which afterward arise.

Fig. 152.—Rudimentary wing of young nymph of Blatta, with the five principal veins developed.

The primitive mode of origin of the wings may, therefore, be best understood by observing the early stages of those insects, such as the Orthoptera and Hemiptera, which have an incomplete metamorphosis. If the student will examine the nymphs of any locust in their successive stages, he will see that the wings arise as simple expansions downward and backward of the lateral edges of the meso- and metanotum. In the second nymphal stage this change begins to take place, but it does not become marked until the succeeding stage, when the indications of veins begin to appear, and the lobe-like expansion of the notum is plainly enough a rudimentary wing.

Graber^[26] thus describes the mode of development of the wings in the nymph of the cockroach:

“If one is looking only at the exterior of the process, he will perceive sooner or later on the sides of the meso- and metathorax pouch-like sacs, which increase in extent with the dorsal integument and at the same time are more and more separated from the body. These wing-covers either keep the same position as in the flat-bodied Blattidæ, or in insects with bodies more compressed the first rudiments hang down over the sides of the thorax. As soon as they have exceeded a certain length, these wing-covers are laid over on the back. However, if we study the process of development of the wings with a microscope, by means of sections made obliquely through the thorax, the process appears still more simple. The chief force of all evolution is and remains the power of growth in a definite direction. In regard to the skin this growth is possible in insects only in this way; namely, that the outer layer of cells is increased by the folds which are forced into the superficial chitinous skin. These folds naturally grow from one moult to another in proportion to the multiplication of the cells, and are not smoothed out until after the moulting, when the outer resistance is overcome.

Fig. 153.—Partial metamorphosis of Melanoplus femur-rubrum, showing the five nymph stages, and the gradual growth of the wings, which are first visible externally in 3, 3b, 3c.—Emerton del.

“As, however, the first wing-layers depend upon the wrinkling of the general integument of the body through the increase in the upper layer, the further growth of the wings depends in the later stages upon the wrinkling of the epidermis of the wing-membrane even, which fact we also observe under the microscope when the new wings drawn forth from the old covers appear at first to be quite creased together. These wing-like wrinkles in the skin are not empty pouches, but contain tissues and organs within, which are connected with the skin, as the fat of the body, the network of tracheæ, muscles, etc. Alongside the tracheæ, running through the former wing-pouches and accompanied by the nerves, there are canals through which the blood flows in and out.

Fig. 154.—Stages in the growth of the wings of the nymph of Termes flavipes: A, young; a, a wing enlarged. B, older nymph; b, fore wing; n, a vein. C, wings more advanced;—D, mature.

Fig. 155.—Wings of nymph of Psocus.

“After the last moult, however, when the supply of moisture is very much reduced in the wing-pouches, which are contracted at the bottom, their two layers become closely united, and afterward grow into one single, solid wing-membrane.

“These thick-walled blood-tubes arising above and beneath the upper and lower membrane of the wing are the veins of the wings; the development of the creased wings in the pupa of butterflies is exactly like that of cockroaches and bugs. The difference is only that the folds of integument furnishing the wings with an ample store of material for their construction reach in a relatively shorter time, that is the space of time between two moults, the same extent that they would otherwise attain only in the course of several periods of growth in the ametabolous insects.”

Fig. 156.—Nymph of Aphrophora permutata, with enlarged view of the wings and the veins: pro, pronotum; sc, mesoscutum; 1ab, 1st abdominal segment.

Ignorant of Graber’s paper, we had arrived at the same result, after an examination of the early nymph-stages of the cockroach, as well as the locusts, Termites, and various Hemiptera. In all these forms it is plainly to be seen that the wings are simply expansions, either horizontal or partly vertical (where, as in locusts, etc., the body is compressed, and the meso- and metanota are rounded downwards), of the hinder and outer edge of the meso- and metanotum. As will be seen by reference to the accompanying figures, the wings are notal (tergal) outgrowths from the dorsal arch of the two hinder segments of the thorax. At first, as seen in the young pupal cockroach (Fig. 152) and locust (Fig. 153, also Figs. 154 and 156) the rudiments of the wings are continuous with the notum. Late in nymphal life a suture and a hinge-joint appear at the base of the wing, and thus there is some movement of the wing upon the notum; finally, the tracheæ are well developed in the wings, and numerous small sclerites are differentiated at the base of the wing, to which the special muscles of flight are attached, and thus the wings, after the last nymphal moult, have the power of flapping, and of sustaining the insect in the air; they thus become true organs of flight.

It is to be observed, then, that the wings in all hemimetabolous insects are outgrowths from the notum, and not from the flanks or pleurum of the thorax. There is, then, no structure in any other part of the body with which they are homologous.

Fig. 157.—Development of wings of Trichoptera: A, portion of body-wall of young larva of Trichostegia; ch, cuticula, forming at r a projection into the hypodermis, m; r, and d, forming thus the first rudiment of the wing. B, the parts in a larva of nearly full size; a, c, d, b, the well-developed hypodermis of the wing-germ separated into two parts by r, the penetrating extension of the cuticula; v, mesoderm, C, wing-pad of another Phryganeid freed from its case at its change to the pupa: b, d, outer layer of the hypodermis (m) of the body-wall; v, inner layer within nuclei.—After Dewitz, from Sharp.

The same may be said of the true Neuroptera, Trichoptera (Fig. 157), the Coleoptera, and the Diptera, Lepidoptera, and Hymenoptera. As we have observed in the house fly,^[27] the wings are evidently outgrowths of the meso- and metanotum; we have also observed this to be most probably the case in the Lepidoptera, from observations on a Tortrix in different stages of metamorphosis. It is also the case with the Hymenoptera, as we have observed in bees and wasps;^[28] and in these forms, and probably all Hymenoptera, the wings are outgrowths of the scutal region of the notum.

With these facts before us we may speculate as to the probable origin of the wings of insects. The views held by some are those of Gegenbaur, also adopted by Lubbock, and originally by myself.^[29] According to Gegenbaur:

“The wings must be regarded as homologous with the lamellar tracheal gills, for they do not only agree with them in origin, but also in their connection with the body, and in structure. In being limited to the second and third thoracic segments they point to a reduction in the number of the tracheal gills. It is quite clear that we must suppose that the wings did not arise as such, but were developed from organs which had another function, such as the tracheal gills; I mean to say that such a supposition is necessary, for we cannot imagine that the wings functioned as such in the lower stages of their development, and that they could have been developed by having such a function.”

Fig. 158.—Changes in external form of the young larva of Calotermes rugosus, showing, in A and B, the mode of origin of the wing-pads: A, newly hatched, with 9 antennal joints, × 8. B, older larva, with 10 joints, × 8. C, next stage, with 11 joints, × 8. D, larva, with twelve joints; the position of the parts of the alimentary canal are shown: v, crop; m, stomach; b, “paunch”; e, intestine; r, heart, × 16 3.—After Fritz Müller, from Sharp.

If we examine the tracheal gills of the smaller dragon-fly (Agrion), or the May-flies, or Sialidæ, or Perlidæ, or Phryganeidæ, we see that they are developed in a very arbitrary way, either at the end of the abdomen, or on the sternum, or from the pleurum; moreover, in structure they invariably have but a single trachea, from which minute twigs branch out;^[30] in the wings there are five or six main tracheæ, which give rise to the veins. Thus, in themselves, irrespective of their position, they are not the homologues of the gills. The latter are only developed in the aquatic representatives of the Neuroptera and Pseudoneuroptera, and are evidently adaptive, secondary, temporary organs, and are in no sense ancestral, primitive structures from which the wings were developed. There is no good reason to suppose that the aquatic Odonata or Ephemerids or Neuroptera were not descendants of terrestrial forms.

To these results we had arrived by a review of the above-mentioned facts, before meeting with Fritz Müller’s opinions, derived from a study of the development of the wings of Calotermes (Fig. 158). Müller^[31] states that “(1) The wings of insects have not originated from ‘tracheal gills.’ The wing-shaped continuations of the youngest larvæ are in fact the only parts in which air tubes are completely wanting, while tracheæ are richly developed in all other parts of the body.^[32] (2) The wings of insects have arisen from lateral continuations of the dorsal plates of the body-segments with which they are connected.”

Now, speculating on the primary origin of wings, we need not suppose that they originated in any aquatic form, but in some ancestral land insect related to existing cockroaches and Termes. We may imagine that the tergites (or notum) of the two hinder segments of the thorax grew out laterally in some leaping and running insect; that the expansion became of use in aiding to support the body in its longer leaps, somewhat as the lateral expansions of the body aid the flying squirrel or certain lizards in supporting the body during their leaps. By natural selection these structures would be transmitted in an improved condition until they became flexible, i.e. attached by a rude hinge-joint to the tergal plates of the meso- and metathorax. Then by continued use and attempts at flight they would grow larger, until they would become permanent organs, though still rudimentary, as in many existing Orthoptera, such as certain Blattariæ and Pezotettix. By this time a fold or hinge having been established, small chitinous pieces enclosed in membrane would appear, until we should have a hinge flexible enough to allow the wing to be folded on the back, and also to have a flapping motion. A stray tracheal twig would naturally press or grow into the base of the new structure. After the trachea running towards the base of the wing had begun to send off branches into the rudimentary structure, the number and direction of the future veins would become determined on simple mechanical principles. The rudimentary structures beating the air would need to be strengthened on the front or costal edge. Here, then, would be developed the larger number of main veins, two or three close together, and parallel. These would be the costal, subcostal, and median veins. They would throw out branches to strengthen the costal edge, while the branches sent out to the outer and hinder edges of the wings might be less numerous and farther apart. The net-veined wings of Orthoptera and Pseudoneuroptera, as compared with the wings of Hymenoptera, show that the wings of net-veined insects were largely used for respiration as well as for flight, while in beetles and bees the leading function is flight, that of respiration being quite subordinate. The blood would then supply the parts, and thus respiration or aëration of the blood would be demanded. As soon as such expansions would be of even slight use to the insect as breathing organs, the question as to their permanency would be settled. Organs so useful both for flight and aëration of the blood would be still further developed, until they would become permanent structures, genuine wings. They would thus be readily transmitted, and being of more use in adult life during the season of reproduction, they would be still further developed, and thus those insects which could fly the best, i.e. which had the strongest wings, would be most successful in the struggle for existence. Thus also, not being so much needed in larval life before the reproductive organs are developed, they would not be transmitted except in a very rudimentary way, as perhaps masses of internal indifferent cells (imaginal discs), to the larva, being the rather destined to develop late in larval and in pupal life. Thus the development of the wings and of the generative organs would go hand in hand, and become organs of adult life.^[33]

The development and structure of the tracheæ and veins of the wing.—The so-called veins (“nervures”) originate from fine tracheal twigs which pass into the imaginal discs. A single longitudinal trachea grows down into the wing-germ (Fig. 147), this branch arising through simple budding of the large body-trachea passing under the rudiment of the wing.

Fig. 159.—Germ of a hind wing detached from its insertion, and examined in glycerine: i, pedicel of insertion to the hypodermis; tr, trachea; b, semicircular pad; e, enveloping membrane; c, bundle of capillary tracheoles; the large tracheæ of the wing not visible; they follow the course of the bundles of tracheoles.—After Gonin.

Gonin states that before the tracheæ reach the wing they divide into a great number of capillary tubes united into bundles and often tangled. This mass of tracheæ does not penetrate into the wing-germ by one of its free ends, but spreading over about a third of the surface of the wing, separates into a dozen bundles which spread out fan-like in the interior of the wing. (Fig. 159). These ramifications, as seen under the microscope, are very irregular; they form here and there knots and anastomoses. They end abruptly in tufts at a little distance from the edge of the wing. A raised semicircular ridge (b) surrounds the base of the wing, and within this the capillaries are formed, while on the other side they are covered by a cellular layer.

Landois, he says, noticed neither the pedicel of the insertion of the wing (i) nor the ridge (b). Herold only states that the tracheæ pass like roots into the wing. Landois believed that they formed an integral part of it. Dewitz and Pancritius used sections to determine their situation.

Fig. 160 will illustrate Landois’ views as to the origin of the tracheæ and veins. A represents the germ of a hind wing attached to a trachea; c the elongated cells, in which, as seen at B, c, a fine tangled tracheal thread (t) appears, seen to be magnified at C. The cell walls break down, and the threads become those which pass through the centre of the veins.

Fig. 160.—Origin of the wings and their veins.—After Landois.

Fig. 161.-Section of the “rib” of a vein: c, cord; b, twig.—After Schaeffer.

The wing-rods.—Semper discovered in transverse sections of the wings, what he called Flügelrippen; one such rib accompanying the trachea in each vein. He did not discover its origin, and his description of it is said to be somewhat erroneous. Schaeffer has recently examined the structure, remarking: “I have surely observed the connection of this cellular tube with the tracheæ. It is found in the base of the wing where the lumen of the tracheæ is much widened. I only describe the fully formed rib (rippe). In a cross-section it forms a usually cylindrical tube which is covered by a very thin chitinous intima which bears delicate twigs (Fig. 161). These twigs are analogous to the thickened ridge of the tracheal intima. I can see no connection between the branches of the different twigs. Through the ribs (rippen) extend a central cord (c) which shows in longitudinal section a clear longitudinal streaking. Semper regarded it as a nerve. But the connection of the tube with the trachea contradicts this view. I can only regard the cord as a separation-product of the cells of the walls.”

Fig. 162.—Parts of a vein of the cockroach, showing the nerve (n) by the side of the trachea (tr); c, blood-corpuscles.—After Moseley.

Other histological elements.—These are the blood-lymph, corpuscles, blood-building masses, and nerves. Schaeffer states that in the immature pupal wings we find besides the large tracheæ, which are more or less branched, and in the wing-veins at a later period, blood-corpuscles which are more or less gorged with nutritive material, and also the “balls of granules” of Weismann, which are perhaps the “single fat-body cells” detected by Semper. Schaeffer also states that into the hypodermal fold of the rudiments of the wings pass peculiar formations of the fat-body and tracheal system, and connected with the fat-body are masses of small cells which by Schaeffer are regarded as blood-building masses.

Fine nerves have also been detected within the veins, Moseley stating that a nerve-fibre accompanies the trachea in all the larger veins in the insects he has examined (Fig. 162), while it is present in Melolontha, where the trachea is absent.